The present invention relates to an optical phase detection method, more particularly to a method of performing optical phase detection through the use of a quadrature component and an in-phase component.
In optical phase shift keying homodyne transmission, it is necessary to perform control to synchronize the phase .theta.l of a local light with the phase .theta.s of a signal light in order to demodulate the phase-modulated signal light. The control can be performed by using a Costas-type optical phase locked loop having an optical 90-degree hybrid unit, as mentioned by A. Schopflin et al. in Electron. Lett. 26,395, 1990.
For the description of an optical phase detection method disclosed in the Japanese Patent Application No. 2-47313 (having a filing date of Mar. 1, 1990, and a publication date of Nov. 11, 1991) made by the two present inventors plus a third inventor, FIG. 4 shows the constitution of a Costas-type optical phase locked loop having a conventional optical 90-degree hybrid unit 1, and FIG. 5 shows the constitution of the unit. The signs .largecircle., , and in FIGS. 4 and 5 denote an S-polarized light component, a P-polarized light component, linear polarization of light, the plane of vibration of which has an angle of 45 degrees to the optical axis of a polarization beam splitter, and circular polarization of light, respectively. The arrows shown by dotted in FIGS. 4 and 5 denote the direction propagation of light. A signal light S1 linearly polarized to have a plane of vibration at an angle of 45 degrees to the optical axis of a polarization beam splitter 1a is entered into it. The splitter 1a splits the signal light S1 into signal lights S2 and S3. A local light L0 emitted from a local light laser light source 2 and linearly polarized is changed into a circularly polarized light L1 by a quarter-wavelength plate 1e. The circularly polarized light L1 is entered into another polarization beam splitter 1c which splits the light into local lights L2 and L3. The P-polarized component S3 of the signal light S1 and the P-polarized component L3 of the local light L1 are combined with each other by a half mirror 1b which is an optical combiner. The S-polarized component S2 of the signal light S1 and the S-polarized component L2 of the local light L1 are combined with each other by another half mirror 1d which is another optical combiner. Lights S4 and L5 and lights S5 and L4, which are obtained through the combining by the half mirror 1b, are received by a balanced receiver 3 so that an output Vb1 is obtained therefrom. Lights S6 and L7 and lights S7 and L6, which are obtained through the combining by the other half mirror 1d, are received by another balanced receiver 4 so that an output light Vb2 is obtained therefrom. As for the hybrid unit 1 shown in FIG. 5, the output light Vb1 is a quadrature component, and the other output light Vb2 is an in-phase component. If the circular polarization of the local light L1 is clockwise with regard to being seen from the quarter-wavelength plate 1e, the phase of the P-polarized component L3 is 90 degrees behind that of the S-polarized component L2 so that the output light Vb1 is the quadrature component proportional to sin (.theta.s-.theta.l), and the other output light Vb2 is the in-phase component proportional to cos(.theta.s-.theta.l) wherein .theta.s and .theta.l denote the phases of the signal light and the local light, respectively. If the circular polarization of the local light L1 is counterclockwise with regard to being seen from the quarter-wavelength plate 1e, the phase of the P-polarized component L3 is 90 degrees ahead of that of the S-polarized component L2 so that the output light Vb1 is the in-phase component proportional to cos(.theta.s-.theta.l), and the other output light Vb2 is the quadrature component proportional to sin (.theta.s-.theta.l).
The optical 90-degree hybrid unit 1 operates so that the polarization of the signal light S1 and that of the local light L1 are made the same as each other by the polarization beam splitters 1a and 1c, and the polarized components of the signal light and the local light are thereafter combined with each other by the half mirrors 1b and 1d, as described above. For that reason, the difference between the phases of the local lights L2 and L3 can be prevented from being affected by the polarization characteristics of the half mirrors 1b and 1d, and are therefore stably kept at 90 degrees.
The signal light S4 and the local light L5, which are outputs from the half mirror 1b, are herein referred to as Q+ component signals. The signal light S5 and the local light L4, which are the other outputs from the half mirror 1b, are herein referred to as Q- component signals different by a phase angle of 180 degrees from the Q+ component signals. The signal light S6 and the local light L7, which are outputs from the other half mirror 1d, are herein referred to as I+ component signals. The signal light S7 and the local light L6, which are the other outputs from the latter half mirror 1d, are herein referred to as I- component signals different by a phase angle of 180 degrees from the I+ component signals.
Since the phase locked loop shown in FIG. 4 employs the balanced receivers 3 and 4 which differentially combine the likewise polarized signals with each other, the half mirrors 1b and 1d which are the output members of the hybrid unit 1 operate as .pi.-hybrid units.
The outputs Vb1 and Vb2(VQ and VI) from the balanced receivers 3 and 4, which are suppressed of intensity noise, are multiplied together by a mixer 5 so as to cancel phase-modulated components by each other to obtain an output V3. The output V3 is applied to a control circuit 6 to obtain an output Vc therefrom. The output Vc is applied as a local light phase control signal to the laser light source 2 to perform feedback control to keep phase synchronization. The signal component of the output Vb2 (VI), which is modulated by the phase .theta.s of the signal light, is demodulated by a code judgment device 7 so that a demodulated signal SO is taken out. Since the local light intensity noises of the outputs Vb1 and Vb2 obtained from the balanced receivers 3 and 4 are suppressed enough by using the optical 90-degree hybrid unit 1 and the differential-synthesis-type balanced receivers in combination, phase detection good in signal-to-noise ratio is enabled.
In the optical 90-degree hybrid unit 1 shown in FIG. 5, the polarized components S2 and S3 of the separated signal light S1 are required to have the same phase as each other in the half mirrors 1b and 1d, and the polarized components L2 and L3 of the separated local light L1 are required to be kept at the phase difference of 90 degrees (which is made by the quarter-wavelength plate 1e) between themselves in the half mirrors. To meet these requirements equations (1-1) and (1-2) wherein lab, 1ad, 1bc, 1cd, .lambda., N1 and N2 denote the optical path length between the polarization beam splitter 1a and the half mirror 1b, that between the splitter and the other half mirror 1d, that between the other polarization beam splitter 1c and the former mirror, that between the latter splitter and the latter mirror, the wavelength of the light, and integers, respectively, need to exist as follows: EQU 1ab=1bc+.lambda..times.N1 (1-1) EQU 1ad=1cd+.lambda..times.N2 (1-2)
However, since errors are made in mounting the polarize tion beam splitters 1a and 1c and the half mirrors 1b and 1d, the equations (1-1) and (1-2) should be replaced with others (2-1) and (2-2) below. EQU 1ab=1bc+.lambda..times.N1+.DELTA.1 (2-1) EQU 1ad=1cd+.lambda..times.N2+.DELTA.2 (2-2)
In these equations, .DELTA.1 and .DELTA.2 denote remainders in dividing the difference 1ab-1bc between the optical path lengths 1ab and 1bc by the wavelength .lambda., and that 1ab-1cd between the optical path lengths 1ad and 1cd by the wavelength, respectively. The remainders determine the relationship between the phases of the signal light and the local light in the optical 90-degree hybrid unit 1, and mean the errors which result in causing a residual phase difference .delta..OMEGA. between the signal light and the local light in synchronous detection as follows: EQU 2.pi.(.DELTA.1+.DELTA.2)/.lambda.=.delta..OMEGA. (3)
It depends on the existence or inexistence of the residual phase difference .delta..OMEGA. whether the polarized components L2 and L3 of the separated local light L1 have the phase difference of 90 degrees between themselves in the half mirrors 1b and 1d. In order to make the quality of optical phase shift keying homodyne transmission good enough to keep the error ratio thereof not higher than 10.sup.-9, the residual phase difference between the signal light and the local light is required to be 0.173 rad or less throughout a phase synchronization system, as mentioned in J. Lightwave Technology, Vol. LT-5, No. 4, Pages 592 to 597, 1987. For that reason, the residual phase difference .delta..OMEGA. which occurs in the optical 90-degree hybrid unit 1 is required to be as follows: EQU .delta..OMEGA.&lt;0.173 (rad) (4)
If the wavelength .lambda. in the equation (3) and the inequality (4) is 1.55 .mu.m, the errors .DELTA.1 and .DELTA.2 in the mounting are required to be as follows: EQU .DELTA.1.times..DELTA.2&lt;0.021 (.mu.m) (5)
Although it is not impossible to mount the polarization beam splitters 1a and 1c and the half mirrors 1b and 1c so as to establish the inequality (5), a highly accurate technique of processing is needed for the mounting and results in a problem of higher cost.
When the phase of the local light is synchronized with that of the signal light through control in the conventional optical phase detection method employing the optical 90-degree hybrid unit 1, the residual phase difference occurs due to the errors in the mounting of the members of the hybrid unit.